This invention relates to an improved system for a tapping operation in a machine tool, and more particularly to improvements in a tapping system in connection with a tool changing mechanism of a machining center which is capable of performing a variety of machining operations automatically.
Machining centers capable of performing milling by contouring control, as well as drilling and tapping in accordance with various fixed cycles, have been developed and are employed in many factories. For the tapping operation a tapper is mounted on the spindle of the machining center, after which a Z-axis feed motor for transporting a workpiece table is actuated to move the workpiece in such a manner that the tip of a tap, which is held by the tapper, approaches the opening of a hole which has been drilled in the workpiece. Then, as the spindle is being rotated, the Z-axis feed motor is allowed to transport the workpiece, thereby allowing the tap to cut a thread into the inner wall of the hole in the workpiece. When a thread of the desired length has been cut, the spindle is rotated in the reverse direction, and so is the Z-axis feed motor to allow withdrawal of the tap from the hole.
This tapping operation performed by the conventional machining center is not controlled with any direct correlation between the spindle motor and the Z-axis feed motor for the workpiece table, and requires that the motors be controlled in speed independently of one another, such as by determining the speed of the Z-axis feed motor in relation to the speed of the spindle. As a result, a number of disadvantages have been encountered in the prior art. Specifically, (1) when the Z-axis feed motor is halted the spindle motor begins to slow down but continues to thread the hole until motor rotation ceases, the result being elongation of the tap; (2) since there are instances where the actual speed of the spindle motor is greater than or less than the set speed, the tap may be elongated when it is thrust into the hole or contracted when it is allowed to withdraw from the hole; (3) if the Z-axis feed motor is rotated in the reverse direction as the spindle motor is slowing down or when it is at rest, the tap is pulled in the opposite direction and elongated; and (4) if the Z-axis feed motor is rotated in the reverse direction while the spindle motor is being accelerated in the reverse direction, the tap is pulled in the opposite direction and elongated. These disadvantages result in damaged screw threads and cause a deterioration in the dimensional precision of the threads. Another serious problem is the damage to the expensive tap as the result of the excessive forces which the tap experiences during the machining operation.